Co-channel interference is an important factor restricting the capacity of a mobile communication system. With the mobile communication system stepping into the 3G era, co-channel interference has evolved into adjacent cell interference with a more apparent restriction against the capacity of the system as a result of emerging single frequency networking. For a Long Term Evolution (LTE) system, a User Equipment (UE) in a current cell may be subject to both interference from a Base Station (BS) in an adjacent cell and interference from a UE in the adjacent cell. For example, the UE in the current cell is subject to interference from the adjacent cell in the downlink as illustrated in FIG. 1, and this applies alike to interference in the uplink.
In order to avoid the foregoing problem of interference and improve the capacity of the system, strict temporal synchronization between respective base stations is required in the LTE system where there is a synchronization error of no larger than 3 μs between any two cell base stations.
At present, synchronization between different cell base stations in a network is achieved in the LTE system using a timing signal of a Global Positioning System (GPS) in a specific implementation as illustrated in FIG. 2 where the respective cell base stations receive a GPS timing signal transmitted from a satellite for synchronization between the cell base stations.
High-rate data services are an important application of the mobile communication system, and the majority of high-rate data access services take place indoors. However it may be very difficult to provide an indoor user with a high-rate data access service through an outdoor macro cell base station alone due to a significant penetrative loss of a radio signal through a building. Therefore a home femto cell base station providing an indoor high-rate data access service has been widely applied. Also due to a penetrative loss, it may be difficult for the femto cell base station deployed indoors to acquire a GPS timing signal directly, thus making it difficult to synchronize among a plurality of femto cell base stations, and a UE may be subject to both interference from an adjacent femto cell base station and interference from the outdoor macro cell base station particularly as illustrated in FIG. 3.
In the LTE system, the cell base station (i.e., the outdoor base station) can be referred to as an eNodeB or simply an eNB, and the femto cell base station can be referred to as a Home-eNodeB or simply HeNB.
Due to an arbitrary deployment location of the HeNB, the HeNB may be synchronized directly with the eNB or synchronized over multiple hops with the eNB or another HeNB synchronized with the eNB during existing air interface synchronization. A synchronization error may be accumulated with an increasing number of synchronization hops in multi-hop air interface synchronization. The HeNBs may interfere with each other during synchronization as illustrated in FIG. 4 where the HeNB1 is listening to synchronization information of the eNB while the HeNB2 is broadcasting its own synchronization signal, and synchronization reliability and accuracy of the HeNB1 at a short distance from the HeNB2 may be degraded seriously. In order to address this problem, such a solution has been proposed in the prior art that the HeNBs all of which periodically keep silent are changed from a transmission status to a reception status when they are silent to listen to a synchronization signal of the eNB, and at this time none of the base stations will transmit any further signal to the UE.
In all the existing LTE air interface synchronization solutions, a Primary Synchronization Sequence (PSS), a Secondary Synchronization Sequence (SSS) or a Common Reference Signal (CRS) of a seed base station (a synchronized base station) is listened to for synchronization information, and although only one Orthogonal Frequency Division Multiplexing (OFDM) symbol in a sub-frame is occupied for the PSS or the SSS, the minimum unit of a sub-frame is required for listening, thus such listening may result in a waste of an air interface resource. Also for the UE, a Physical Broadcast Channel (PBCH) is located in the same sub-frame as the sequence of SSS, and the HeNB listening to synchronization information transmits no further signal to the UE so that the UE may lose both the sequences of PSS and SSS and broadcast information and thus be influenced greatly. Furthermore the scenario of multi-hop synchronization has not been taken into account for air interface synchronization in the prior art.